Conventionally, measurement, of a flow velocity of water flow in a water body (e.g., a river) may be done in a number of ways. For example, a current meter, such as a Price type current meter, a propeller-type current meter or the like may be employed to detect a velocity of a fluid (e.g., water) in the water body. The current meter may be disposed in the water body for measuring the flow velocity at one or more depths (e.g., 20%, 60% or 80% of a depth of the water body). However, the current meter may not function properly when the flow velocity of the water is higher than, for example, 4 meters per second.
Another way to perform the measurement is to dispose a floating object (e.g., a buoy or an object made by expanded polystyrene) on a surface of the water body, and to calculate the flow velocity of the water on the surface based on a displacement of the floating object within a time period. This method may be employed when it is determined that the flow velocity of the water is higher than 4 meters per second. However, retrieval of the floating object is difficult, and leaving the floating object in the water body may cause pollution of the environment.
A further way is to perform the measurement without contacting the water and involves use of a surface velocity radar (SVR) that may be handheld or placed stationarily. It is noted that, once the velocity of the fluid is known, a volumetric flow rate (also known as discharge) may be calculated by multiplying the velocity of the fluid by an area of a cross-section of a part of the water body where the measurement is performed, and an early warning of flood may be outputted according to the volumetric flow rate. However, the SVRs are generally handheld and operated by human inspectors instead of planted at each of the rivers mainly due to cost. As a result, continuous measurement and monitoring of each of the rivers becomes difficult because of limited man power and also potential safety concerns stemming from hazardous weather conditions (e.g., flooding caused by a typhoon).